The Pentagon announced that it awarded a contract worth as much as $7.4 billion to aerospace giant Lockheed Martin Corp. to upgrade the Air Force's fleet of problem-plagued F-22 Raptor fighter jets.

The announcement said that the Bethesda, Md., company would “add new capabilities and enhance the performance" of the aircraft.

Lockheed referred all questions about the contract to the Air Force, which said it could not provide details about the upgrade announced Friday.

The F-22, which cost an estimated $412 million each, is the military’s most expensive fighter jet and known to be its most advanced. Yet the stealthy, supersonic plane has never been used in Iraq, Afghanistan, or any other combat zone since it entered service for the military in 2005.

The F-22 has experienced seven major crashes with 2 fatalities.

Last May, the entire fleet of F-22s was taken out of service after a dozen incidents occurred since April 2008 in which pilots' oxygen was cut off.

After a government safety investigation grounded the jets for more than four months, the Air Force returned all 170 of the F-22s to flight operations in September. The Air Force did not disclose whether engineers had identified the problems with the oxygen system or whether it had been fixed.

About a month after reentering service , several F-22s were temporarily grounded in October after an incident at Langley Air Force Base in Virginia.